The present application relates to a method and apparatus for measuring chromatic dispersion in optical waveguides for optical communication, and more specifically to such method and apparatus using Raman effect.
Chromatic dispersion occurs in optical waveguides due to differences of propagation speeds at various wavelengths. Therefore, the optical pulse signal expands as it travels along the waveguide, deforming the waveform and causing crosstalk between adjacent channels.
It is desirable to measure the chromatic dispersion in optical waveguides in order to provide proper dispersion compensation. Various methods of measuring chromatic dispersion in optical fibers have been proposed.
In U.S. Pat. No. 4,752,125 two types of optical pulses having different wavelengths are transmitted through an optical fiber. The group velocity delay time is measured as the flight time difference between the two optical pulses. The total amount group velocity dispersion, or chromatic dispersion, is measured from the group velocity delay time and the wavelength difference.
In U.S. Pat. No. 5,189,483, Inagaki describes another pulse delaying method based on transmitting a laser pulse generated by Raman oscillation through a sample fiber. At the output end of the fiber, a reference wavelength light and an object wavelength light are received and a delay time of the object wavelength light relative to the reference wavelength light is measured as a factor of a chromatic dispersion of the sample fiber. Notably, the Raman gain takes place in the measuring instrument, and not in the waveguide under test.
Other than the above described time-of-flight techniques, other methods of measuring chromatic dispersion include the soliton method and the interferometer method. More commonly, chromatic dispersion is measured by determining the phase shift of a modulated signal transmitted along an optical fiber. There are two phase-shift methods used for measuring chromatic dispersion. In the first method, an input signal is modulated and is transmitted through the optical fiber for which chromatic dispersion is to be measured. The phase of the transmitted signal is compared to the phase of a fixed reference signal, which for example, may have been simultaneously transmitted through the fiber or may have been used to modulated the input signal. The phase difference of different input wavelengths relative to the fixed reference wavelength provides the group velocity delay, which is subsequently used to calculate the chromatic dispersion. In the second phase shift method, commonly referred to as the differential phased shift method, the dispersion is measured directly, without calculating the group velocity delay. In particular, the dispersion is calculated from a the phase difference between a first modulated signal at a first wavelength and a second modulated signal at a second wavelength. The phase differences are obtained using electronic phase detectors.
Phase-shift methods are advantageously more accurate. Moreover, a simple filter or monochromator can be used as a wavelength selector. Disadvantageously, phase shift methods require communication between the input and output ends of the device under test (DUT) (e.g., the optical fiber). It is desirable to measure chromatic dispersion without requiring phase information, i.e. without extracting phase properties of the signal at both ends of the waveguide which requires complex electronics.
In U.S. Pat. No. 5,724,126, Nishi et al. describe a method and apparatus for measuring a distribution of zero-dispersion wavelengths. In this method and apparatus an optical pulse and a pump signal are launched into an optical fiber and the amplification of the pulse signal resulting from modulation instability induced by the pump signal is detected from the back-scattered light waveform of the pulse signal. Advantageously, the apparatus and method proposed by Nishi et al. only requires measurements at one end of the optical fiber. Disadvantageously, the apparatus and method proposed by Nishi et al. does not provide information regarding group delay and/or wavelength dependence of the dispersion.
It is an object of the instant invention to provide a novel method and apparatus for measuring group delay and/or chromatic dispersion.
According to the invention, a modulated pump signal and a probe signal are propagated through a waveguide medium and the ratio of the modulation power of the pump signal to the modulation power of the output signal is determined and related to the chromatic dispersion.
The Raman gain varies along the fiber spatially and temporally due to the modulated pump signal. These variations get transferred to the probe signal. The efficiency of the transfer between the pump modulation and the probe modulation at the output of the waveguide depends on the difference in group velocity of the signals, which relates to the chromatic dispersion between the two wavelengths. The amplitude of both signal modulations are measured and related to the dispersion.
In one embodiment the probe signal is provided by a tunable laser diode. In another embodiment, the probe signal corresponds to amplified spontaneous emission generated by the modulated pump signal due to spontaneous Raman scattering in the fiber.
According to the present invention, there is provided a method of measuring group delay in an optical waveguide, the method comprising the steps of: a) providing a modulated pump signal to the optical waveguide; b) allowing the pump signal to propagate within the optical waveguide so as to generate gain within the waveguide, the gain having the modulation of the pump signal impressed thereon; c) varying the modulation frequency of the pump signal; d) measuring a frequency response of the modulated gain while the modulation frequency of the pump signal is varied; and determining the group delay from the frequency response of the modulated gain.
According to the present invention, there is provided a method for measuring chromatic dispersion of an optical waveguide having an input end and an output end, the method comprising the steps of: a) inputting a modulated pump signal into the input end of the waveguide to generate Raman gain in the waveguide, b) inputting a probe signal into the input end of the waveguide, the probe signal having a wavelength that is within Raman gain band characteristic of the waveguide, c) combining the pump signal and the probe signal at the input end of the waveguide, d) impressing the modulation of the pump signal on the probe signal through temporal and spatial Raman gain modulation in the waveguide, e) varying the modulation frequency of the pump signal, f) measuring frequency response of the probe signal at the output end of the waveguide while the modulation frequency of the pump signal is varied, g) determining the group delay from the frequency response of the probe signal, h) varying the wavelength of the probe signal, i) repeating steps a) to g) for different probe wavelengths to determine a relationship of group delay and wavelength, and j) determining the chromatic dispersion of the waveguide from said relationship.
In accordance with another aspect of the invention, there is provided an apparatus for measuring chromatic dispersion of a waveguide having an input end and an output end, the apparatus comprising: a Raman pump source operatively coupled to the input end of the waveguide, the Raman pump source for providing a pump signal to the input end; a modulator coupled to the Raman pump source, the modulator for modulating the Raman pump signal; and, a detector operatively coupled to the output end of the waveguide, the detector for measuring, at the output end of the waveguide, a frequency response of a probe signal simultaneously propagating through the waveguide with the modulated Raman pump signal.
The apparatus may further comprise combining means for combining the pump signal and a probe signal at the input end of the waveguide, and means for separating the pump signal and the probe signal at the output end of the waveguide.
The modulator may be an external intensity modulator operatively connected to the pump signal source. It may be embodied by an electrical modulator or by an optical modulator.